In recent years, in order to enhance protection or beauty of a display in flat panel display devices such as mobile phones and personal digital assistants (PDA), a thin sheet-shaped cover glass is arranged on the front surface of a display such that the area of the cover glass is wider than an image display part. Reduction in weight and reduction in thickness are required in such a flat panel display device, and to achieve the requirement, a thickness of a cover glass used for display protection is required to be decreased.
However, when the thickness of a cover glass is decreased, strength of the cover glass is decreased, and in some cases, the cover glass itself breaks by drop during using or carrying a display device, and there was a problem that an original role of protecting a display device cannot be performed.
For this reason, in the conventional cover glass, scratch resistance of a cover glass was increased by chemically strengthening a glass sheet to form a compressive stress layer on the surface thereof (JP-A-2011-105598). In order to increase mechanical strength of a glass substrate, it is necessary to impart large compressive stress to a glass surface (JP-A-2008-115071). Furthermore, in order to increase mechanical strength, it is proposed to deepen a compressive stress layer (JP-A-2002-174810).
On the other hand, in order to improve cutting properties of a glass, Japanese Patent No. 4289931 describes a method for manufacturing a chemical strengthened glass, comprising after ion-exchanging, conducting a dipping treatment at a temperature higher than that of a first dipping treatment, and then conducting a heat treatment. Furthermore, in order to improve cutting properties of a glass, Japanese Patent No. 4289927 describes a method for keeping a glass at a temperature higher than a temperature of a first dipping treatment after ion-exchanging.
Moreover, JP-A-54-83923 describes an ion-exchange strengthening method comprising forming a surface compressive stress layer on a glass surface layer by an ion-exchange treatment, subjecting the glass to a heat treatment at a temperature of 480 to 600° C. to move the compressive stress layer to the inner part of the glass from the glass surface, and then ion-exchanging the glass to form the compressive stress layer on the glass surface layer.